1. Field of the Invention
The present invention relates to a radar apparatus based on a spread spectrum technique.
2. Description of the Related Art
UWB (ultra wide band) radar apparatuses, which employ a spread spectrum technique, have been developed. In the spread spectrum technique, data are spread in a wide band with use of a certain code sequence so as to reduce influences of interference from other communication systems. In a case where, for instance, the radar apparatus is an on-vehicle radar apparatus, the radar apparatus can detect, with high precision, whether or not an obstacle (a target) is present ahead of the vehicle, a distance between the vehicle and the target, and a relative velocity to the target.
In the radar apparatus, in order to determine a distance to the target, a despread code for use in despreading a received wave reflected from the target is delayed with respect to a transmission code by a predetermined amount in accordance with the distance. Therefore, when the amount of delay is varied under the influence of changes in the characteristics of a semiconductor device—which constitute the radar apparatus—due to a change in temperature and/or power supply variation, a shift occurs between a phase of the transmission code and that of the despread code. As a result, a correlation property between the transmission code and the despread code varies. In some cases, such a change in the correlation property gives rise to an error in detection of a distance to the target.
FIG. 1 is a block diagram showing a schematic structure of a radar apparatus, which employs a conventional spread spectrum technique. In FIG. 1, reference numeral 1 denotes a code generator for generating a spectrum spreading code and a despread code. Reference numeral 2 denotes a spread/transmission processor for modulating a transmission signal with the spectrum spreading code generated by the code generator 1 and for conducting transmission processing. Reference numeral 3 denotes a transmission antenna for transmitting the signal having been subjected to the spreading processing.
Reference numeral 4 denotes a reception antenna for receiving an electric wave having been transmitted from the transmission antenna 3, reflected by a target 13, and returned, in other words, for receiving a reflected wave 14 reflected by the target 13. The reception antenna 4 also receives a direct wave 15 transmitted from the transmission antenna 3. Reference numeral 15 denotes a reception processing section for conducting demodulation processing of the signal received by the reception antenna 4. Reference numeral 6 denotes a stepped delay section for delaying the despread code generated by the code generator 1 stepwise. Reference numeral 7 denotes a despreading processor for conducting despreading processing of the received signal with the despread code having been delayed by the stepped delay section 6 and for further effecting A/D conversion. Reference numeral 8 denotes a correlation value detector for detecting a correlation value from the thus despread and A/D converted data. Reference numeral 9 denotes a target detector for conducting a variety of types of processing, such as determination of a distance to the target, determination of a reception intensity, determination of a relative velocity between the target and the own vehicle, determination of identification of the target on the basis of the detected correlation value.
The result of the processing by the target detector 9 is displayed on an unillustrated display, thereby calling attention to a driver; alternatively, the result may be input to an ECU of the vehicle, to thereby be utilized in a variety of driving controls.
In the apparatus shown in FIG. 1, timing of receiving a direct wave, which is directly transmitted by the antenna 3 and received by the antenna 4, involves a predetermined amount of delay with respect to a timing of generating code by the code generator 1. Such delay depends on the physical structure, arrangement, and the like, of equipment. Accordingly, the despread code generated by the code generator 1 must be delayed by the same amount. A fixed delay section 10 generates the delay for the above purpose (i.e., initial delay).
FIG. 2 is a timing chart for explaining operations of the apparatus shown in FIG. 1, and particularly showing how a distance to the target is detected. In FIG. 2, (a) shows a code sequence for spread spectrum generated by the code generator 1 and (b) shows a despread code sequence delayed stepwise by the stepped delay section 6. For the purpose of synchronization with the received code, the fixed delay section 10 gives a predetermined initial delay to the despread code sequence (b).
The despread code sequence (b) is formed by delaying transmission codes (1), (2), (3), . . . , (n) in increments of, e.g., one clock cycle, by the stepped delay section 6. In FIG. 2, (c) shows a received code sequence of a direct wave; (d) shows a code sequence of a wave reflected by a target spaced from the radar apparatus by a distance corresponding to one clock cycle; (e) shows a code sequence of a wave reflected by a target spaced from the radar apparatus by a distance corresponding to two clock cycles.
As shown in FIG. 2, in a case of despreading the received code sequence (c) of the direct wave by use of the despread code sequence (b), a code 1 shows a strong correlation; and the remaining codes do not show strong correlations because phases of the received codes and those of the despread codes are deviated. Similarly, the code sequence (d) of the wave reflected by the target spaced a distance corresponding to one clock cycle shows a strong correlation with a code 2 of the despread code sequence (b). Furthermore, the code sequence (e) of the wave reflected by the target spaced a distance corresponding to two clock cycles shows a strong correlation with a code 3 of the despread code sequence (b).
Accordingly, when the received wave (f) is despread with the despread code sequence (b) and then a correlation value of each code is detected, it is detected whether or not a target for producing a reflected wave is present at each of distances as shown in (g) of FIG. 2. More specifically, presence of a target is indicated when a strong correlation is found among correlation properties at distances corresponding to 0 m, 0 m +1 clock cycle, 0 m+2 clock cycles, . . . .
As described above, the UWB radar apparatus employing the spread spectrum technique correlates the despread code having been delayed stepwise by the step delay section with the received code; and determines a presence of a target and a distance to the target on the basis of the correlation properties. Therefore, a phase of the despread code and that of the received code must be in accurate synchronization. When for some reason the phase relationship between the both codes differs from a predetermined one, proper correlation strength cannot be obtained in detection of correlation values as shown in (g) of FIG. 2. As a result, a correct distance to the target cannot be detected.
However, the characteristics of a semiconductor device, which constitutes the radar apparatus, are easily changed under the influence of a change in temperature and/or power supply variation. In addition, degrees of the variations are not uniform among devices. Consequently, in the despreading processor, a phase shift occurs between the received code and the despread code.
FIG. 3 shows a phase shift (a delay difference) between a despread code and a received code arising from a change in temperature and/or power supply variation. When a signal speed is low, such a small phase shift is of little consequence. However, the phase shift exerts great influence on a device using high-speed signals such as the UWB radar apparatus, resulting in a great change in a correlation property between the received code and the despread code.
When a delay difference as shown in FIG. 3 is found before use of the radar apparatus, the difference can be absorbed by adjusting a value of the fixed delay section 10. However, when such a delay difference is caused by a change in temperature and/or power supply variation during usage, it cannot be absorbed by the fixed delay section 10.
Example references of the invention include JP-A-2002-290273, JP-A-2000-310675, and JP-A-Hei.9-211111. However, these references simply disclose the technical level of general spread spectrum radar apparatuses at the time the invention was made, and neither indicate nor present features of the invention.